Pulsed NdYAG lasers are widely used in industrial processes such as welding, cutting and marking. Care has to be taken in these processes to ensure that the plasmas generated by the laser does not interfere with the incoming laser pulses. The relatively low pulse repetition rates (6 kHz) at high peak powers that are achievable in a NdYAG laser have led to their wide application in laser machining.
Fibre lasers are increasingly being used for materials processing applications such as welding, cutting and marking. Their advantages include high efficiency, robustness and high beam quality. Examples include femtosecond lasers for multiphoton processing such as the imaging of biological tissues, Q-switched lasers for machining applications, and high-power continuous-wave lasers. Their disadvantage is their relatively low energy storage capacity as compared to NdYAG lasers.
In many applications, fibre lasers need to compete with the more mature diode pumped solid state lasers. In order to do so, much greater optical powers need to be achieved, with high reliability and lower cost.
Fibre lasers are typically longer than diode-pumped solid state lasers, and this leads to non-linear limitations such as Raman scattering becoming problematical. It would be advantageous to have fibre lasers that are shorter.
Fibre lasers are typically pumped with diode lasers in bar or stack form, or by many single-emitter diodes that are combined together. Fibre lasers can be core pumped, in which case the pump radiation is guided by the core of the active fibre, or cladding pumped, in which case the pump radiation is guided by the cladding of the active fibre. The active fibre in a cladding-pumped fibre laser needs to be longer than in a core-pumped fibre laser in order to absorb the pump radiation. This is because there is less interaction between the pump radiation and the core in a cladding pumped fibre laser than in a core-pumped fibre laser. Typically, the length of active fibre needs to be longer by the ratio of the cladding cross-sectional area and the core cross-sectional area in order to absorb the pump radiation and provide the necessary output energy. Cladding pumped fibre lasers that have been described in the prior art have inner claddings that are either rectangular, have flats machined on them, have a shape such as a polygon, or are asymmetric.
U.S. Pat. No. 4,815,079 discloses a cladding pumped fibre having a rectangular cladding and another cladding pumped fibre having a circular cladding with an offset core. These designs increase the coupling of pump radiation guided by the cladding and the fibres core. The fibres do not have the combination of a central core and a uniform cladding diameter, which make them difficult to cleave and couple radiation in connectors.
U.S. Pat. No. 5,533,163 discloses a cladding pumped fibre having an inner cladding in the form of a non-rectangular, convex polygon so that the propagating pump energy is induced to form an essentially uniform radiation field in which the various radiation modes comprising the pump energy are isotropically distributed. The fibres do not have the combination of a central core and a uniform cladding diameter, which make them difficult to cleave and couple radiation in connectors.
U.S. Pat. No. 5,864,645 discloses a circular cladding pumped fibre having at least one flat extending along its length to break circular symmetry and to set up chaotic ray behaviour. Such a fibre can be awkward to cleave, the fibre tending to twist when clamped leading to undesirable angled cleaves.
None of the above mentioned prior art shapes provides high coupling of cladding modes with the core modes whilst also combining a substantially regular geometry with curved outside edges that is suitable for cleaving, incorporating into optical fibre connectors, and coupling radiation from substantially round sources.
An aim of the present invention is to provide an apparatus for providing optical radiation that reduces the above aforementioned problem.